1. Field of the Invention
The present invention relates to a position measuring system which is particularly suitable for determining the absolute position of two objects which can be moved with respect to each other. The present invention furthermore relates to a method for operating a position measuring system.
2. Discussion of Related Art
A category of known position measuring systems for determining absolute positions includes an incremental track extending in the measuring position, as well as an absolute track arranged parallel to it, on the scanned scale, and has a serial absolute coding, for example. A correspondingly constructed position measuring system is described in DE 195 05 176 A1, for example. For an exact position determination it is necessary to synchronize the resulting position-dependent scanning signals from the two tracks with each other. This is critical in particular when these signals clearly differ in the order of magnitude of the respective local resolution. Additional problems result when scale tapes are used on the scale, since their parallel alignment with the measuring direction is sometimes not always optimally possible. Measurement errors result if, for example, a twisting of the scale occurs around an axis, which is oriented perpendicularly in relation to the scale surface.
To solve these problems it was therefore proposed in German Patent Application 199 62 278.7 of Applicant to generate at least two incremental signals of different resolution from the scanning of a suitable incremental track. Besides the appropriate design of the scanned incremental track, an appropriate design of the respective detector arrangements in the scanning unit is also required for this. Two suitable detector variations are proposed for this purpose by the above mentioned patent application, by which the discussed problem, which arises in the course of possible twisting of the scanning unit, or of the scale, around an axis which is oriented perpendicularly in relation to the scale surface, can be solved.
Besides undesirable twisting around this axis, in the course of measuring operations pitch movements of the scanning unit or of the scale can occur around axes, which are oriented in the scale plane, or parallel with the scale plane, and perpendicularly in relation to the measurement direction. Such pitch movements can also result in errors during the synchronization of scanning signals with different local resolutions, in particular if the scanning signals from different tracks show a different pitch behavior.